Polymers have been used in various technical fields. A polymer material such as a plastic film is formed by using a melt or solution of a polymer. A process of forming a polymer material comprises dissolving a polymer in a solvent to form a solution, forming a polymer material by using the solution, and drying the formed material by evaporating the solvent.
The solvent of a polymer is a liquid that can dissolve a polymer at a required concentration. The solvent also requires safety and an appropriate boiling point for evaporating the solvent from a formed polymer material. Recently, the solvent particularly requires safety of the human body and the environment. Therefore, it is now rather difficult to find out an appropriate solvent in liquids that can dissolve a polymer.
For example, methylene chloride has been used as a solvent of cellulose triacetate. However, the use of hydrocarbon halides such as methylene chloride has recently been restricted severely to protect the global environmental conditions. Further, methylene chloride may cause problems in the working environment.
On the other hand, acetone is a widely used organic solvent. Acetone has an appropriate boiling point (56.degree. C.). Further, acetone has few problems on the human body and the global environmental conditions, compared with the other organic solvents. However, cellulose triacetate has a poor solubility in acetone. Cellulose triacetate can be swelled in acetone, but is scarcely dissolved in acetone.
J. M. G. Cowie et al. report in Makromol., Chem., 143 (1971) 105-114, that cellulose acetate having a substitution degree in the range of 2.70 (acetic acid content: 60.1%) to 2.80 (acetic acid content: 61.3%) is dissolved in acetone by a specific process. The process comprises the steps of cooling the cellulose acetate in acetone to a temperature of -80 to -70.degree. C., and warming it to obtain 0.5 to 5 wt. % solution of the-cellulose acetate in acetone. The method of cooling a mixture of a polymer and a solvent to obtain a solution is hereinafter referred to as a cooling dissolution method.
The solution of cellulose acetate in acetone is also reported by K. Kamide et al., Textile Machinery Society, Vol. 34, 57-61 (1981). The report (written in Japanese) is entitled "Dry spinning process using acetone solution of triacetyl cellulose." In the report, the cooling dissolution method is applied to the art of fiber spinning. The experiments shown in the report examine the mechanical strength, the dyeing property and the cross sectional profile of the fiber obtained by the cooling dissolution method. In the report, 10 to 25 wt. % solution of cellulose acetate is used to form a fiber.